Magnetic recording on hard disk drives is accomplished by magnetizing regions of alternating magnetic polarity on a rotating disk through the use of a magnetic recording head. Traditional magnetic recording heads may include a standard electromagnetic coil to generate the alternating magnetic fields required for writing.
Innovations in magnetic recording have been focused on shrinking the size of the magnetized regions of hard disk drives, thereby increasing the number of magnetized regions (or “recording density” per unit area) and the amount of information that can be stored on a given hard disk drive. As recording density of hard disk drives increase, the tendency of the magnetized regions of the hard disk drives to become destabilized by ambient thermal energy may increase. The phenomenon whereby a small magnet flips rapidly back and forth in response to ambient thermal energy is called superparamagnetism, and the limit on magnetic recording density due to superparamagnetism is often referred to as the superparamagnetic limit. To increase the superparamagnetic limit, the magnetic energy density within the magnetized regions may be increased by using materials with higher magnetic energy density. However, a higher magnetic field may be needed to write on these materials—typically one that is greater than that which can be generated with a traditional magnetic recording head.
To generate the higher magnetic field needed to write on materials with higher magnetic energy density, heat assisted magnetic recording (HAMR) and thermally assisted recording (TAR) has been developed. In HAMR and TAR, a nanoscale heating source is added to the magnetic recording head. This heat source is used to heat the recording medium and temporarily reduce the switching field that the recording head must provide to write on the recording medium. After the writing process is complete, the recording medium cools and is no longer subject to thermal destabilization.
The heated spot size should be well below that which can be achieved through the far field focusing of light, as limited by diffraction. Consequently, metallic guiding structures, such as near field transducers (NFTs), are typically used in HAMR systems. NFTs may exploit plasmonic electromagnetic modes to localize optical energy to a sufficiently small spot. NFTs may be illuminated directly, via waveguides or focused light, or they may be attached to resonant structures that can generate or absorb incident radiation and drive the NFTs. Conventional resonant structures that have been used to drive NFTs have been metallic, but the dissipation in the metals limited the energy storage capability of the metallic resonator. NFTs have also been placed directly on the output facet of a laser, such that near field optical coupling can be used to drive the NFTs.